The Global Positioning system (GPS) is a time-synchronized space-based satellite system that broadcasts spread spectrum codes from a nominal constellation of 24 earth-orbiting satellites. GPS uses Code Division Multiple Access (CDMA) to simultaneously broadcast multiple codes on each GPS frequency. Since the standard GPS constellation consists of 24 satellites, 24 codes are normally broadcast simultaneously on each GPS frequency.
Because each satellite broadcasts a unique code, and because the codes have poor cross-correlation properties, it is relatively easy to use matched replica correlation processing to extract any particular code from the rest. In matched replica processing, a replica of a code sequence is compared with the received signal. The replica will correlate with the received signal only if the received signal contains the sequence of codes that match the replica and only when the replica is correctly aligned with the code sequence.
The GPS satellite constellation is continuously monitored, with ephemeris and clock corrections broadcast so that receiving equipment can locate GPS satellites at any time and make corrections necessary to account for errors in each satellite""s clock. The distance from the satellite to the receiver can be determined from the location of a satellite, the time that the satellite""s coded message was transmitted, and the nominal time the message was received. If codes are received from four or more GPS satellites, then the receiver""s position and the error in the receiver""s clock can be estimated. It is necessary to ensure that the receiver""s clock is synchronized with the satellite constellation""s clock in order to accurately measure the elapsed time between a satellite""s code sequence transmission and reception of that code sequence by a GPS receiver.
GPS can provide highly accurate position estimates. However, the GPS signal frequency is approximately 1.5 GHz and its power level at the receiving antenna is xe2x88x92160 dBW. The high frequency and low received power of the GPS signals restrict the use of GPS to locations where the receiver""s antenna has a clear, line-of-sight view of the requisite number of satellites. The requisite number of satellites is normally four, but can be less if some form of aiding is used. For example, if altitude aiding is used, it may be possible to obtain a GPS solution with three satellites. This line-of-sight restriction degrades GPS performance in buildings, in vehicles, under foliage, in areas with steep terrain where the horizon is blocked by mountains or high buildings, or other places where the GPS antenna does not have an unobstructed view of the sky.
One way to mitigate these problems is to add pseudolites on the ground to augment the space-based GPS satellite constellation. Pseudolites, or pseudo-satellites, function like GPS satellites but are located on the Earth""s surface rather than in orbit about the Earth. In the context of this invention, the term xe2x80x9cpseudolitexe2x80x9d means any transmitter that broadcasts coded spread spectrum signals (e.g., pseudo-random code sequences) that can be used to determine the distance between the transmitter and the receiver, i.e., ranging signals. A pseudolite can broadcast on one or more of the GPS satellite frequencies, or it can broadcast on a separate frequency. If a pseudolite broadcasts on a GPS frequency, it can mask the GPS broadcasts, particularly when the receiver is in close proximity to the pseudolite. Broadcasting on a separate frequency adds cost to the receiver because the receiver must have the frequency bandwidth to receive and process both the GPS and pseudolite signals.
Like GPS satellites, pseudolites broadcast repeating pseudo-random code sequences that a receiver on the ground can process to determine the elapsed time between code transmission and code reception. A pseudolite receiver can determine the distance between a pseudolite and the receiver from (1) the location of a pseudolite, (2) when the pseudolite transmits its code sequence, (3) nominally when the pseudolite receiver receives the code sequence, and (4) the speed of light. The pseudolite receiver processes the pseudolite transmissions in exactly the same way a GPS receiver processes the GPS satellite transmissions. A dual GPS/pseudolite receiver can concurrently process and apply range measurements from satellites and/or pseudolites to determine the receiver""s location. That is, the receiver can use GPS measurements, pseudolite measurements, or a combination of GPS and pseudolite measurements to determine its own location.
A ground-based pseudolite has two major advantages over a space-based satellite. First, the ground-based pseudolite does not move, and its position can therefore be determined to any achievable measurement accuracy. Achievable accuracy is a function of the amount of time one spends making the measurement: using dual frequency GPS carrier tracking, centimeter accuracy can be achieved. Second, the pseudolite""s output power can be arbitrarily large, allowing the signal to be received in areas where GPS is often blocked.
When multiple pseudolites broadcast on a common frequency, they suffer from what is known as the xe2x80x9cnear-far problem.xe2x80x9d A pseudolite that is too near a receiver can block reception of signals from distant pseudolites. This occurs when the received power level from the near-field pseudolite is much higher than the received power level from the far-field pseudolite, thereby masking the weaker signal. When pseudolites broadcast at the same frequency as the space-based GPS constellation, they can, when near enough to the receiver, mask the satellite signals.
The near-far problem does not arise between satellites in orbit because the relative distance from each satellite to an observer on earth is about the same, no matter where the observer moves on the surface of the earth. For example, the GPS constellation is in orbit approximately 26,600 Km above the earth""s center of mass. If the satellites are in a 20,200 Km orbit above the surface of the earth (26,600 Km orbit above the center of the earth), then a satellite that is directly-overhead is 20,200 Km away from an observer standing on the Earth""s surface, which is as close as an in-orbit GPS satellite can be to an observer. A GPS satellite that is on the horizon, the greatest distance from which a signal from an in-orbit GPS satellite can be received on earth, will be approximately 25,800 Km away from the observer. Therefore, the distance between an observer and any two GPS satellites never varies by more than 28%, with the associated received signal levels differing by 2 dB or less.
This is not the case with pseudolites on earth. Consider two pseudolites, one 10 Km from the receiver and another 1 Km from the receiver. If both pseudolites are broadcasting at the same power level, there will be a 100-fold difference between the received power levels of the two pseudolites, since, assuming spherical signal spreading between the pseudolite and receiver, received power is inversely proportional to the square of the distance.
When a pseudolite broadcasts on the GPS frequency, the near-far problem can be mitigated if the pseudolite only broadcasts part of the time. If, for example, the pseudolite broadcasts for 100 msec and then is off for 900 msec, it will have a 10% duty cycle. That is, the pseudolite broadcast will only interfere with the GPS broadcast 10% of the time. A good GPS receiver will be able to maintain lock on the GPS constellation if the pseudolite duty cycle is short enough (typically 10% or less). The length of a pseudolite""s duty cycle is constrained in two ways. First, it must be long enough to allow the receiver to receive and process the pseudolite data stream. Second, it must not be so long that it prevents the receiver from receiving and processing data streams from the GPS constellation or other pseudolites. From a practical standpoint, using the GPS frequencies for pseudolite broadcasts has two major shortcomings. One is the near-far problem and the associated interference with the GPS transmissions. The second is the fact that the GPS spectrum is protected.
The near-far problem between a pseudolite and the GPS constellation can be eliminated if the pseudolite broadcasts on a different frequency than the GPS constellation. However, when multiple pseudolites broadcast on the same frequency (GPS frequency or otherwise), the near-far problem will exist between pseudolites. This near-far interference can be eliminated through the use of Time Division Multiple Access (TDMA) transmissions. In a TDMA system, each source transmits during a different time interval. The present invention interleaves multiple pseudolite broadcasts using time division multiplexing. In this way, no two satellites that are in proximity to one another broadcast at the same time. Multiple pseudolites can use the same broadcast time-slot when the geographic distance between pseudolites is great enough and pseudolite power levels are low enough to prevent one pseudolite from interfering with another.
When time division is used to multiplex pseudolite transmissions at an off-GPS frequency, pseudolites can be used to augment the GPS constellation without interfering with the GPS constellation or interfering with one another. In addition, a network of pseudolites can be established to work independently of GPS. For example, pseudolites can be placed at selected wireless telephone base-stations, with pseudolite transmissions first time-synchronized, then interleaved using TDMA. A GPS-like receiver in the wireless handset could then use pseudolite broadcasts to calculate the location of the wireless handset in the same way that a GPS receiver uses GPS satellite broadcasts to calculate the GPS receiver""s location.
Pseudolites have been employed as a means of improving the reliability of GPS location capabilities in various vehicle location systems. In U.S. Pat. No. 5,311,194 to Brown, a pseudolite is employed to transmit differential corrections to GPS satellite code and carrier measurements to a broadband GPS receiver on board an aircraft, and to provide additional code and carrier measurements to assist in a navigation solution in an approach and landing system for aircraft. Brown discloses the use of a single pseudolite broadcasting at a frequency offset from the L1 GPS frequency in order to prevent interference with the satellite navigation system. However, Brown does not recommend the use of time-slotted transmission because such a signal format does not allow contiguous carrier phase measurements of the pseudolite signal and affects the use of the signal as a communication link.
U.S. Pat. No. 5,646,630 to Sheynblat describes a system for differential navigation of an autonomous vehicle, employing a plurality of satellites, a plurality of ground transmitters, and a base station. The ground transmitters disclosed by Sheynblat do not suffer the near-far problem because they do not have receiving antennas or satellite tracking capabilities. The autonomous vehicle does receive ground transmitter signals, and although Sheynblat discloses TMDA communication links, TDMA broadcasts are not used to mitigate any near-far problem experienced by the autonomous vehicle.
U.S. Pat. No. 5,301,188 to Kotzin describes the use of TDMA timeslots to solve the near-far problem in a TDMA cellular network, where a relatively distant telephone subscriber might experience interference from the tail of one time frame boundary overlapping the beginning of another time frame boundary. Kotzin proposes to reduce this problem by allocating time slots nearest a frame boundary to subscribers nearer the site. Kotzin does not address TDMA transmissions in a GPS environment, the invention instead being directed toward a mechanism for shared-carrier frequency-hopping.
The present invention is a method and apparatus comprising at least three binary code signal sources, with at least one signal source being a pseudolite which transmits binary code signals on at least one radio frequency not used by the Global Positioning System. Each pseudolite transmits signals according to a time division multiplexing system. The invention further comprises at least one mobile receiver, typically a wireless telephone, which receives binary code signals. Each mobile receiver is associated with a signal time processor and a location processor. Each signal time processor determines binary code signal arrival times at an associated mobile receiver, and each location processor determines the location of an associated mobile receiver from binary code signal arrival times and binary code signal source location information.
The present invention comprises a method and apparatus for augmenting the GPS with ground-based satellites, or pseudolites, for the principal purpose of locating wireless telephones, using time division multiplexing, or Time Division Multiple Access (TDMA), to interleave coded spread spectrum or pseudo-random code sequence pseudolite transmissions. When pseudolites broadcast concurrently on the same frequency or in the same frequency band, signals from pseudolites near a receiver can block reception of signals from pseudolites farther from the receiver. The present invention avoids this near-far problem by employing time division multiplexing so that, within a given geographic area, no two pseudolites broadcast at the same time on the same frequency. Additionally, a pseudolite broadcasting on a GPS frequency can block GPS signals. GPS signal blocking is avoided by confining pseudolite broadcasts to non-GPS frequencies. The present invention also comprises a network of pseudolites broadcasting TDMA-synchronized signals and operating independently from the GPS constellation.
A first object of the present invention is to allow multiple pseudolite transmissions to occur without interference. Standard GPS applications rely entirely on continuous CDMA transmissions to broadcast all codes simultaneously. The present invention interleaves pseudolite broadcasts with a TDMA transmission scheme to prevent the near-far problem that can occur when a receiver is closer to one pseudolite than it is to another pseudolite.
A second object of the present invention is to expand the area in which a mobile receiver can receive geolocation signals. The GPS signal is often blocked or masked by foliage, buildings, geographic features, and other structures. In the present invention, a network of time-synchronized pseudolites in selected geographic locations augments the GPS or operates independently of the GPS, expanding geolocation signal coverage and improving accuracy. Time division multiplexing allows pseudolites that are in geographic proximity to one another to broadcast at the same time, and also allows increases in pseudolite transmission power levels without creating signal conflicts. Higher transmission power levels improve signal reception within and behind interfering structures. In addition to locating wireless telephones, this invention is also applicable to location of other objects, in conjunction with or independent of GPS, such as cars and trucks, railroad cars, or any other object. Expanded effective location area and improved accuracy support the FCC""s E-911 initiative, improving system capability for locating mobile phones making 911 emergency calls, and also support other value-added services such as location-based billing, fleet management, and Intelligent Transport System (ITS) applications.
A third object of this invention is to allow placement of pseudolites at existing wireless base-stations, independent locations, or a combination of independent and base-station sites. Pseudolites can often use existing wireless broadcast facilities, or can be sited independently. Higher pseudolite transmission power levels extend signal range, allowing the use of fewer pseudolites to cover a geographic area.
An fourth object of this invention is to use frequency spectrum currently allocated for wireless communications for pseudolite broadcasts. The pseudolite broadcasts if this invention either use dedicated spectrum or share the spectrum with other applications by spreading the pseudolite transmission across the frequency band, avoiding interference with GPS signals while efficiently utilizing bandwidth resources.
These and other objects are accomplished in accordance with the preferred embodiment of the present invention, in which a network of pseudolites broadcasts CDMA signals interleaved by a TDMA system on a non-GPS radio frequency. Pseudolites may be located at existing wireless sites or at independent locations. The pseudolites may synchronize signal transmissions with the GPS system. Adjacent pseudolites broadcast at different times so as to eliminate near-far signal interference between pseudolites. A mobile receiver, typically a cellular telephone, receives pseudolite transmissions, and may also receive GPS transmissions. A time of arrival processor, which may be embedded in the mobile receiver or located elsewhere, determines the distance of each transmission source from the mobile receiver. A location processor, which likewise may be embedded in the mobile receiver or located elsewhere, combines source distance measurements with source location information to find the location of the mobile receiver. The location processor may access source location information in a database within the mobile receiver, or location information may be encoded in transmissions. Once the mobile receiver location is determined, the location may be reported to a remote location.